This invention is generally directed to acoustic transducers which utilize a Helmholtz resonator driven by a piezoelectric element. More specifically, this invention is directed to an acoustic transducer of this type having a design which facilitates the removal of a liquid from the Helmholtz chamber. The ability to remove liquids and contaminates from the Helmholtz chamber is particularly important where the transducer is mounted to a printed circuit board (PCB) that will undergo conventional wave soldering and cleaning processes.
The use of piezoelectric elements including monomorph and bimorph benders in loudspeakers is known. The piezoelectric element converts an electrical signal representative of a desired sound into mechanical vibrations that result in the generation of sound.
In U.S. Pat. No. 4,413,198 issued to Jonathan Bost, a piezoelectric transducer apparatus is disclosed in which a piezoelectric driver element is simultaneously coupled to a first and second Helmholtz resonator. Each resonator is tuned to a different frequency and has a single output port disposed intermediate one wall of the resonator. Thus, the interior cavity of each Helmholtz resonator is accessible only by the single port.
A transducer including a piezoelectric driver element coupled to a Helmholtz resonator is disclosed in U.S. Pat. Nos. 3,873,866 and 3,982,142 issued to Ralph Goble. The Helmholtz resonator disclosed in these patents consists of an enclosed cylindrical chamber having a plurality of radially aligned circular holes formed in the cylindrical sidewall of the chamber. These holes are spaced apart from the top and bottom walls which define the Helmholtz chamber.
Piezoelectrically driven Helmholtz resonators designed for mounting to a PCB are available. These resonators are generally cylindrical and are mounted at one end to the PCB. A single aperture centered in the end of the transducer opposite the PCB provides a single tuned port for the Helmholtz chamber. Since the PCB is normally disposed in a horizontal plane during a wave soldering operation, the port faces generally upward. Thus, the opportunity exists during this operation for contaminates to enter the Helmholtz chamber via this port.
Following the wave soldering step, PCB's are normally immersed in or sprayed with a cleaning solution in order to remove flux and other residues which may have been deposited during the wave soldering operation. During this cleaning operation it is likely that the cleaning liquid will enter the Helmholtz chamber via the port. Once the cleaning solution or other contaminates enter the chamber, it is very difficult to remove such contaminates from the chamber. Even if the transducer and PCB are oriented such that the port faces down, the inherent vacuum which exists in the chamber with the port obscured by the cleaning liquid prevents, or at least inhibits, the flow of the liquid from the chamber.
One solution to this problem with Helmholtz chambers having a single port consists of placing a sticker over the port during the wave soldering and cleaning operations. This sticker which is secured by an adhesive must be removed from the transducer following the cleaning operation. This solution unfortunately creates other problems. A substantial amount of labor is required to properly position the sticker over the port and later remove the sticker. Also, the additional handling required to place and remove the sticker increases the chance that adjacent components on the PCB may become misaligned or damaged. Furthermore, the inadvertent failure to remove the sticker will cause the transducer to be substantially inoperative and will be perceived by the customer as a defective feature of the product.
In light of the above explanation, it is believed to be apparent that there is an existing need for a piezoelectric driven Helmholtz transducer which is fully compatible with standard PCB processing steps, i.e., it will not need special protective procedures.